The present invention relates to a method of controlling the fuel injection in an internal combustion engine comprising a piston, which passes to and fro in a cylinder between an upper and a lower dead centre, and an injection nozzle, which is disposed in the cylinder. The method comprises the steps whereby a primary injection of fuel is realized in the cylinder, which fuel is ignited and causes the piston to be moved in the direction of the lower dead centre in the cylinder, and whereby a post-injection of a fluid is realized in the cylinder.
A combustion process in which the fuel is injected directly into the cylinder and is ignited by increased temperature and pressure in the cylinder is commonly referred to as the diesel process. When the fuel is injected and burns in the cylinder, a turbulent mixing takes place of combustion gases present in the cylinder with the burning fuel. The combustion of the fuel/gas mixture in the cylinder generates heat, which heat generation causes the gas in the cylinder to expand and thereby causes the piston to move within the cylinder. Depending on a large number of parameters, such as the injection pressure of the fuel, the quantity of exhaust gases recycled to the cylinder, the time of injection of the fuel and the turbulence and temperature prevailing in the cylinder, different efficiency values and engine emission values are obtained.
Conventional internal combustion engines which work according to the diesel process have relatively high values in terms of discharged emissions, such as soot particles. During the expansion, regions having an inadequate excess of air are found locally in the cylinder, resulting in incomplete combustion of the fuel injected into the cylinder. Emissions in the form of soot particles are thereby generated, which accompany the exhaust gases during the exhaust stroke.
It is previously known to reduce the formation of soot particles by injecting the fuel early in or in advance of the expansion stroke or working stroke, whilst at the same time seeking to delay ignition of the fuel, so that the fuel gets to be vaporized and is mixed prior to ignition with gases present in the cylinder. Methods therefore exist for reducing the content of emissions from a conventional engine. There are, however, limitations with these known methods, which in extreme cases make the engine inoperable.
When the piston, during the working stroke, is moved towards the lower dead centre, the pressure and temperature in the cylinder fall. During the expansion, the turbulence or the remixing of gases, fuel and formed soot particles has also been shown to be comparatively low in the cylinder, and especially in a peripheral region close to the cylinder wall. In overall terms, this leads to a reduction in the oxidation of formed soot particles during the expansion, which unoxidized soot particles accompany the exhaust gases of the internal combustion engine during the exhaust stroke.
One object of the present invention is to control the fuel injection in an internal combustion engine such that the abovementioned drawbacks with the prior art are eliminated and such that the soot particle content in the exhaust gases of the internal combustion engine is as low as possible.
A further object of the invention is to increase the oxidation of soot particles formed in the cylinder in order thereby to reduce the number of soot particles in the exhaust gases of the internal combustion engine.
This is achieved by virtue of a method of the type stated in the introduction, in which the injection pressure of the fluid in the post-injection is higher than the injection pressure of the fuel in the primary injection.
As a result of the post-injection, fuel, gas and soot particles in regions in which a turbulent or agitating motion has ceased are made to recirculate and be mixed, which enables and accelerates the oxidation of soot particles. By controlling the post-injection such that the fluid is injected at a higher pressure than the injection pressure of the fuel in the primary injection, regions in which turbulence and remixing have ceased are more quickly reached, thereby creating vigorous remixing in the said regions.